Question: In physics class, the girl to boy ratio is $4$ to $8$. If there are $20$ girls, how many boys are there?
A ratio of $4$ girls to $8$ boys means that there is a group of $4$ girls for every group of $8$ boys. If there are $20$ girls, then there are $5$ groups of $4$ girls each. Because we know the ratio is $4:8$ , there must be $5$ groups of $8$ boys each. There is a total of $40$ boys in physics class.